In fabricating cables and harnesses containing a plurality of wires, it is desirable to provide fluid blocks to prevent the passage of fluids, such as water, along the cables. The problem of fluid passage arises in various industrial and commercial applications where cables are used, such as the automotive and telecommunications fields. In cable assemblies used in automobiles, for example, it is important to prevent moisture from migrating along the wires in the cable to various electrical components in different parts of the automobile. It is also desirable to prevent the passage of moisture, fumes and noise through the cable from the engine compartment to the passenger area.
Various techniques have been employed to deal with this problem. In the automotive field, wire harness assemblies are sometimes arranged with "drip loops" which consist of U-sections of the wires hanging down so that water passing along the wires will drip off at the bottom of the U-section. Obviously this is only a partial solution to the problem.
One desirable technique is to provide a packing or sealant around the wires in a protective rubber sleeve, which is designed to form a complete fluid block when heated. This technique and recent variations of the packing or sealant element is described in detail in U.S. Pat. Nos. 4,972,042 and 5,378,879 issued to Seabourne et al. and Monovoukas, respectively, and U.S. patent application Ser. No. 08/806,183, filed Feb. 25, 1997 by Rodkey et al., each assigned to the same assignee as the present application and incorporated herein by reference in their entirety.
The Seabourne patent discloses the use of fusible polymeric sealant, such as hot-melt adhesives or thermosetting adhesives, in a heat-shrinkable covering, tubing, or sleeve surrounding the cable wires. The application of heat to this assembly causes the adhesive to melt and surround the wires, forming a block upon cooling. Epoxy sealant may also be utilized, in which the application of heat facilitates curing and formation of a permanent fluid block in the cable.
Since this technique requires the application of heat to the assembly in a controlled manner to provide a satisfactory blocking structure, both the temperature of the assembly and the heating time must be carefully monitored. Excessive temperatures can cause damage to the cable wires or insulation, as well as the protective covering and sealant. On the other hand, if the heating temperature is too low, the blocking seal may not form completely and the block will be ineffective to prevent fluid passage. Ideally the heating should be uniform throughout the cable block to avoid hot spots and cold spots in the sealant.
To help provide the necessary uniform induction heating process, the Monovoukas patent discloses a technique of distributing ferromagnetic particles within a packing or sealant material, such as polymeric sealant. When ferromagnetic particles are added to this electrically non-magnetic and non-conductive material, it may be heated by magnetic induction heating by exposing it to high frequency alternating electromagnetic fields. The temperature of the ferromagnetic particles increases until the particles reach their Curie temperature, and the particles are self-regulating at that temperature. As disclosed in the Monovoukas patent, this technique may be used in the fabrication of sealant blocks in wire cable and harness assemblies.
The Rodkey et al. patent application modifies the wire harness structure of the Seabourne patent by providing a wire harness in a comb-like structure. The comb harness eliminates a cannonballing effect which occurs when three wires nest together, creating interstices which form a leak path between them.
FIG. 1 illustrates an example of a wire bundle 2 having a harness comb sealant 4 and a heat recoverable tubing 6 similar to those described above. Once the bundle is heated by the appropriate induction coil heating process, the structure will provide a bundle block for preventing any liquids or fumes from passing through the wire bundle.
Induction heating is a widely used heating method for applications requiring precise heating control. U.S. Pat. No. 5,630,958 by Stewart, Jr. et al., having the same assignee of the present invention, and which is incorporated herein by reference in its entirety, discloses a conventional multi-turn induction coil for heating a wire bundle block assembly as illustrated in FIG. 1 and described above. More specifically, this patent describes the use of a multi-turn "U" shaped coil having a movable flux concentrator to enhance the uniformity of heating in a load received laterally to the coil structure. An alternating current source having a high frequency (MHz) drives the coil to generate a high magnetic flux density in the load.
When multi-turn coils are employed in heating loads as illustrated in FIG. 1, the tubing 6 takes the most amount of time to recover. This duration is usually two to three times the duration needed to melt the comb adhesive 4. Consequently, although the coil will heat the load to produce the desired bundle block, because of the extra heat provided to the load, other components of the load exposed to the magnetic field may be damaged.
For example, in a typical wire bundle, copper wires coated with insulation are present and the copper is inductively heated. The copper is not self-regulating in temperature because it does not have a Curie temperature like ferromagnetic materials. As a result, the copper continues to heat as power is continuously applied, the insulation surrounding the copper continues to heat due to heat generated by the copper, and wire becomes damaged.
The window period in which adequate heating of the article occurs without damage to components may be extremely small or nonexistent. This problem is addressed in commonly owned U.S. patent application Ser. No. 08/403,032, filed Mar. 13, 1995, U.S. Pat. No. 5,672,290 entitled "Power Source and Method for Induction Heating of Articles" by Levy et al., which is assigned to Raychem Corporation, the same assignee of the present invention, and is incorporated herein by reference in its entirety. This technique decreases the induction heat generated in the load wires by adjusting the power levels provided to the coil after a predetermined time. The time and power level adjustment are predetermined by factors such as the bundle size, and the Curie temperature of the ferromagnetic particles used in the bundle comb and tubing.
Through experimentation by the applicants, single-turn coils have proven to be very efficient in heating the adhesive comb 4 and heat-shrinkable tubing 6 by electromagnetic induction. FIG. 2 illustrates a single-turn inductive coil 10. The coil 10 includes an interior diameter 12 and a length 14 comparable to the portion of the load (not shown) to be heated. By driving the single turn coil 10 with an AC source having a low frequency in the kHz to low MHz range, the tubing recovery time is significantly reduced (it approaches the time needed to melt the combs), which in turn, increases the installation window for the product by reducing the total bundle block install time.
The increase in performance is attributable to the existence of a much more uniform longitudinal and radial magnetic flux density in the coil as compared to the multi-turn coil. However, as shown in FIG. 3, as a result of the dynamics of tubing recovery, the ends of the tubing 6 "flare up" 16 and then "flip back" 18 over the tubing 6 when the wire bundle 2 is heated in the coil 10 (the wire bundle has a comparable size to the coil). This effect is due to the low magnetic flux density applied to the two ends of the heat-shrinkable tubing 6 causing a slower recovery rate at the ends of the tubing 6 rather than at the middle portion.